Continuous Theta-Burst Stimulation on the Left Posterior Inferior Frontal Gyrus Perturbs Complex Syntactic Processing Stability in Mandarin Chinese

Abstract The structure of human language is inherently hierarchical. The left posterior inferior frontal gyrus (LpIFG) is proposed to be a core region for constructing syntactic hierarchies. However, it remains unclear whether LpIFG plays a causal role in syntactic processing in Mandarin Chinese and whether its contribution depends on syntactic complexity, working memory, or both. We addressed these questions by applying inhibitory continuous theta-burst stimulation (cTBS) over LpIFG. Thirty-two participants processed sentences containing embedded relative clauses (i.e., complex syntactic processing), syntactically simpler coordinated sentences (i.e., simple syntactic processing), and non-hierarchical word lists (i.e., word list processing) after receiving real or sham cTBS. We found that cTBS significantly increased the coefficient of variation, a representative index of processing stability, in complex syntactic processing (esp., when subject relative clause was embedded) but not in the other two conditions. No significant changes in d′ and reaction time were detected in these conditions. The findings suggest that (a) inhibitory effect of cTBS on the LpIFG might be prominent in perturbing the complex syntactic processing stability but subtle in altering the processing quality; and (b) the causal role of the LpIFG seems to be specific for syntactic processing rather than working memory capacity, further evidencing their separability in LpIFG. Collectively, these results support the notion of the LpIFG as a core region for complex syntactic processing across languages.

Since the accuracy of the easiest task, that is, the simple sentence processing, did not reach 90%, we assumed that none of the tasks showed a ceiling effect.Moreover, two-way repeated measures ANOVA was performed for comparing either the 3 (Complex, Simple, Word List) or the 4 sequence types (SR, OR, Simple, Word List).Results showed that only the main effect of sequence types was significant [for 3 sequence types: F(2, 62) = 52.472,p < .001,ηp 2 = .629;for 4 sequence types: F(3, 93) = 34.502,p < .001,ηp 2 = .527].Post hoc paired comparisons showed that the accuracy of simple sentence was significantly higher than those of both complex (SR and OR) sentence and word list processing conditions [for 3 sequence types: ts(31) ≥ 8.670, pbonfs < .001,Cohen's ds ≥ 1.275; for 4 sequence types: ts(31) ≥ 6.172, pbonfs < .001,Cohen's ds ≥ .828].Nevertheless, no statistical accuracy differences could be found between the complex (SR and OR) sentence and word list processing conditions [Complex vs. Word List: t(31) = .068,pbonf = 1.000,Cohen's d = .009;SR vs.

CTBS ON LPIFG IN CHINESE SYNTACTIC PROCESSING 2
Word List: t(31) = 1.860, pbonf = .435,Cohen's d = .254;OR vs. Word List: t(31) = -2.148,pbonf = .238,Cohen's d = .329].The accuracy difference pattern was also consistent with the results reported in a previous fMRI study (Liu et al., 2023: Supporting Information 1.2), in which the difficulty of the word list processing (i.e., the working memory) task was well matched with that of the complex sentence (either SR or OR) processing task.Therefore, the cTBS effect difference between complex sentence and word list processing conditions was more likely to be attributed to the nature of the task per se rather than the difficulty difference between these two tasks.

Figure S1
Raincloud plots for accuracy of the 3 and 4 sequence types under both cTBS and sham conditions Note.W: word list processing (i.e., working memory task), colored in green; S: simple sentence processing, colored in orange; C: complex sentence processing, colored in purple.
SR: complex sentence with subject relative clause embedded processing, colored in pink; OR: complex sentence with object relative clause embedded processing, colored in purple.

Behavioral Data Analysis Blind to the Conditions
Given that we intended to compare the differences ("Δ": cTBS -sham) between the conditions (either 3 or 4 sequence types), experimenters were not bind to the sessions (cTBS or sham) during the data analyses as described in the main text.A concern was whether the blindness to the data/conditions would have biased the results.Therefore, as an ad-hoc test, we invited another experimenter who was totally blind to the conditions to re-analyze the CV(coefficient of variation) data which showed significance between the three/four conditions.In particular, the cTBS condition was masked as the "a" condition, and the sham condition was labeled as "α", and the experimenter was asked to subtract the CV data of one condition from the other according to his own will.The experimenter obtained the ΔCV from the subtraction of "a -α", and the repeated measures ANOVA showed exactly the same results to those reported in the main text, thus indicating that in the present study, the blindness to the conditions had little bias to the current analyses as well as the related results.

Behavioral Data Analyses of the Whole Set of Participants
We originally recruited 33 participants in total.However, there was one more participant who underwent the sham session firstly, resulting in the unbalanced numbers of the two stimulation types for each session: "17 sham + 16 cTBS" for the first session, and "16 sham + 17 cTBS" for the second session.Therefore, before the actual data analyses, data of one of the participants who attended the sham session as the first session was randomly discarded.Now we added this participant's data for the ΔCV analysis of the whole set of participants, and found the similar result in the comparison among the 4 sequence types [F(3, 96) = 3.510, p=.018, ηp 2 = .099]as reported in the main text, further indicating that the cTBS effect (reflected by the differences of ΔCV between the sequence types) should be robust.

Analysis of the Session Order Effect
To further assess the potential influence of session order effects, we divided the participants into two groups: Group 1 (subjects who received real cTBS first, then followed by "sham") and Group 2 (subjects who received "sham" first, followed by real cTBS).Then we performed 3 (sequence types: Complex, Simple, and Word List) × 2 (group: Group 1 and Group 2) and 4 (sequence types: SR, OR, Simple, and Word List) × 2 (group: Group 1 and Group 2) ANOVAs separately on the indices Δd', ΔRT, and ΔCV.In these ANOVAs, the sequence types were used as a with-subject variable while the group served as a between-subject variable.Descriptive statistics were generated for all indices and summarized in Table S2 to Table S4.
The results for ΔRT showed no significant group differences in both the 3 sequence Therefore, the session order was unlikely to affect the stimulation effect differences among the sequence types.Note.Group 1 contained subjects who received the real cTBS condition followed by the "sham" condition and Group 2 contained subjects who received the "sham" condition followed by the real cTBS condition.

Table S2
Sequence type Δd' under Group 1 and Group 2 type ΔRT under Group 1 and Group 2